Significantly increased production of H2 on ZnIn2S4 under visible light through co-deposited CoWO4 and Co3O4

Much work has focused on metal chalcogenides as photocatalysts for water splitting. Herein ZnIn2S4 (ZIS) has been deposited with 10 wt% CoWO4, followed by mixing with 3 wt% Co3O4. Reaction was conducted under a 420 nm LED lamp in the presence of triethanolamine (TEOA). In terms of the amounts of H2 evolved at 2 h (nH2h), Co3O4/CoWO4/ZIS, Co3O4/ZIS, and CoWO4/ZIS were more active than ZIS, respectively, by factors of approximately 9.2, 7.0, and 2.5. In addition, Co3O4/CoWO4/ZIS was more photostable than ZIS. The trend for the nH2h not only coincided with those for proton reduction and water (TEOA) oxidation on a dark electrode, respectively, but also matched the reduced emission intensity of the solid in ambient air. However, for water oxidation on a photoanode, Co3O4/ZIS was less active than CoWO4/ZIS. Then the larger nH2h of Co3O4/ZIS than CoWO4/ZIS is due to Co3O4 being more active than CoWO4 for catalyzing proton reduction. Based on the solid band edge potentials, a possible mechanism is discussed. The photoholes of ZIS recombine with the photoelectrons of Co3O4 (CoWO4), improving the efficiency of charge separation, and hence increasing the rates of proton reduction on ZIS and TEOA oxidation on Co3O4 (CoWO4), respectively. Because of the combined Z-scheme pathways, Co3O4/CoWO4/ZIS is more photoactive than others.